B. 1,200 kg-m/s Momentum is defined as the product of mass and velocity. So let's see what the momentum is for each spaceship. Spaceship 1 has a mass of 300 kg and a velocity of 0 m/s, giving 300 kg x 0 m/s = 0 kg-m/s Spaceship 2 has a mass of 300 kg and a velocity of 4 m/s, giving 300 kg x 4 m/s = 1200 kg-m/s Since we're looking for their combined value, add the 2 figures together. 0 kg-m/s + 1200 kg-m/s = 1200 kg-m/s So the answer is option B. 1,200 kg-m/s